1. Technical Field
This disclosure relates to image forming apparatuses, and more particularly, to an image forming apparatus having a structure where a conveyance belt configured to carry and convey a recording medium is stretched around plural rollers and at least one roller among the plural rollers works as a driving roller which rotationally drives the conveyance belt.
2. Description of the Related Art
An inkjet printer has been known as an image forming apparatus wherein ink drops are jetted from a jet opening of a head so that an image is formed on the recording medium. In this inkjet printer, the ink drops jetted from the jet opening directly reach the surface of a paper so that an image is formed. Because of this, in order to realize a high quality image, it is necessary to improve the precision of a reaching position (position of contact) of the ink drop on the paper. As a method for improving the precision of the reaching position of the ink drop, the distance between the head and the paper is kept constant or paper conveyance is performed at high precision. As a method for carrying the paper at high precision, an image forming apparatus wherein a conveyance belt for conveying the paper to a position facing the head is electrostatically and evenly charged so that the paper is electrostatically adhered is disclosed in Japanese Laid-Open Patent Application Publications No. 4-2011469 and No. 9-254460.
FIG. 1 is a schematic view showing a main part of the related art image forming apparatus. As shown in FIG. 1, the related art image forming apparatus has a carriage 230 and a conveyance belt 231. The carriage 230 has a head 209. The conveyance belt 231 is stretched around a driving roller 232 and a tension roller 233 with a proper tension.
In addition, this image forming apparatus has a charging roller 237 and a pushing roller 236. A surface of the conveyance belt 231 is electrostatically charged by the charging roller 237. The paper is pushed onto the conveyance belt 231 at a position facing the driving roller 232 by the pushing roller 236.
The paper conveyed from a paper feeding tray or a manual tray (not shown) to the conveyance belt 231 is pushed onto the conveyance belt 231 by the pushing roller 236 so that the paper is electrostatically adhered to the conveyance belt 231. The paper electrostatically adhered to the conveyance belt 231 is conveyed to a position facing the head 209 by the conveyance belt 231.
A frictional force between the driving roller 232 and the conveyance belt 231 may be changed due to foreign particles coming in between the driving roller 232 and the conveyance belt 231 so that a slippage between the driving roller 232 and the conveyance belt 231 may happen. If the slippage happens, the paper electrostatically adhered to the conveyance belt 231 cannot be conveyed to the position facing the head 209 at good precision. As a result of this, the reaching position of the ink drop may be shifted so that a high quality image may not be formed.
For solving this problem, a pressing force of the pushing roller 236 applied to the conveyance belt 231 may be increased so that the slippage between the driving roller 232 and the conveyance belt 231 may be prevented. However, in this case, if the pressing force of the pushing roller 236 applied to the conveyance belt 231 is increased, it is difficult to for the paper to pass between the pushing roller 236 and the conveyance belt 231 so that paper jamming may happen.
Such a problem happens in not only the inkjet type image forming apparatus but also an image forming apparatus wherein a toner image is transferred onto a paper carried by the conveyance belt.